1. Field of the Invention.
The present invention relates to the optimized coupling of pump radiation into the gain medium of a laser and to systems including such lasers. Particular aspects of the present invention relate to the use of one or more arrays of semiconductor diodes to side pump a high absorption, high gain solid state gain medium such as neodymium yttrium vanadate.
2. Description of the Related Art.
Solid-state lasers are a class of lasers that use as a gain medium a crystalline, glass or other solid material as a host for an optically excitable material, such as an ion of a rare earth element like neodymium. The crystalline, glass or other solid host material acts as a matrix fixing the optically excitable material in place. For illustrative purposes, it is convenient to consider a common solid-state laser system that uses yttrium aluminum garnet (YAG) doped with neodymium ions as a gain medium. This system is usually identified as Nd:YAG. Application of pump radiation to the Nd:YAG solid-state gain medium excites Nd3+ dopant ions within the YAG host material to metastable states that have lifetimes which allow the population of the states to become inverted in such a manner that optical transitions of the dopant ions from the excited metastable states can be used to establish laser action. Some solid-state laser systems use a broad-band optical radiation source such as a xenon flash-lamp or other type of flash-lamp to provide pump radiation to the gain medium to excite the dopant ions to their desired metastable states. Flash-lamps have the advantages of high available power levels and comparatively low cost. Nevertheless, the use of broad-band, poorly-focused pump radiation is undesirable because pump radiation is absorbed in portions of the gain medium other than within the resonant modes of the laser. Pumping portions of the gain medium that are not within the resonant modes does not result in laser action and generates heat that must be dissipated, and so can limit the range of usable pump intensities.
A second undesirable aspect of the inefficiency of flash-lamp excitation is that the broadband radiation of flash-lamps is not well matched to the task of pumping the dopant ions within the resonant modes to the desired excitation levels. That is, within flash-lamp pump radiation there are many photons with more energy than is necessary to excite the neodymium ions of the solid-state gain medium to their desired metastable states. When these too energetic photons are absorbed by the dopant ions, the excess optical energy absorbed by the neodymium ions beyond what is necessary to place the neodymium ions in their desired metastable state is dissipated at least partially through thermal excitation of the solid-state gain medium. In other words, the excess energy within the radiation absorbed into the resonant modes heats the gain medium. To a lesser extent, a similar phenomenon occurs for photons within the radiation that are insufficiently energetic to excite the neodymium ions to their desired states. Materials suitable as solid-state gain media tend to have imperfect crystal structures with comparatively broad absorption bands and with significant levels of defect-mediated absorption for comparatively low-energy photons. The fraction of the flash-lamp radiation absorbed that is insufficiently energetic to excite the dopants to their desired metastable states is also typically dissipated at least partially through heating of the gain medium.
For a variety of reasons, dissipating heat and avoiding unnecessary heat generation are important considerations for solid-state gain media. Solid-state lasers are generally used in high power applications, with corresponding high levels of input pumping power and high levels of heating of the gain medium. The cooling of solid-state gain media, whether using coolant based refrigerators or using solid-state refrigerators, is not wholly satisfactory because solid-state gain media are generally poor thermal conductors so that the interiors of solid-state gain media are in poor thermal communication with the surfaces through which cooling may occur. There is consequently a limit to the rate at which heat can be extracted from the pumped portion of the gain medium by cooling. Thus, the inherent properties of solid-state gain media and solid-state lasers are such that heat will build up in the gain media of these lasers. To make matters worse, solid-state gain media tend to be particularly ill suited to excessive heating conditions. Solid-state gain media are typically susceptible to mechanical failure under high thermal stress. Consequently, heating of the solid-state gain medium is frequently a limit on system performance.
The various problems associated with flash-lamp pumping of solid-state gain media have historically limited the applications for which solid-state lasers were considered appropriate. A major advance for the practical application of solid-state lasers proceeded from the maturation of semiconductor diode lasers as excitation sources for pumping the solid-state gain media. Through the use of sophisticated manufacturing techniques, semiconductor lasers are now available operating at wavelengths that are very close in energy to the excitation energies most suitable for many solid-state laser systems. These semiconductor lasers exhibit desirable mode quality and operate at sufficiently high energies to be useful for pumping solid-state gain media. Commercially available semiconductor lasers with output powers of twenty watts or greater generally consist of a linear array of discrete laser diode emitters formed on a single chip. These semiconductor laser bars can be made to have any of a variety of desirable output wavelengths and, for example, can have an output wavelength near 808 nanometers, which is convenient for pumping a solid-state laser system like Nd:YAG. Furthermore, these diode lasers are tunable during use over a sufficient range (xc2x13 nanometers) to allow the diode laser bar to be matched to a particular solid-state gain medium. Tuning of semiconductor lasers is accomplished by adjusting the operating temperature of the semiconductor laser. The availability of semiconductor laser output wavelengths matched to the energy levels being pumped is important because it allows the efficient pumping of the solid-state gain medium with very little of the pump radiation directly generating heat within the gain medium. Energy level efficient pumping of the gain medium significantly reduces the amount of heat generated in the solid-state gain medium due to imprecise pumping.
Although semiconductor lasers can efficiently pump solid-state gain media, the maximum power output of semiconductor lasers is lower than is desirable for many solid-state laser applications. Consequently, it is important to collect as much of the semiconductor laser""s pump light as is possible within the gain volume of the solid-state gain medium to take advantage of as much of the semiconductor laser""s output intensity as is practical. The commonly used Nd:YAG gain medium is not a strong absorber of the wavelength of light most appropriate to pumping the neodymium ions within the YAG matrix. When a semiconductor laser is used to side pump a Nd:YAG gain medium, a substantial fraction of the pump light can pass entirely through the relatively small cross-sectional dimensions of the Nd:YAG gain medium without being absorbed. It is consequently very desirable that the pump light output by the semiconductor laser be directed along the longest axis of solid-state gain media like Nd:YAG and that the optical axis of the resonant cavity extend along a direction nearly parallel to that longest axis to increase the amount of semiconductor laser pump light that is absorbed within the resonant modes of the gain medium. Because of this, many solid-state lasers pumped by semiconductor lasers have used what is called an xe2x80x9cend-pumpedxe2x80x9d configuration. Typical solid-state lasers include a resonant cavity defined by front and back mirrors with a generally cylindrical or rectangular-prism shaped gain medium placed within the resonant cavity so that the end faces of the gain medium are located along an optical axis that encounters or faces respective ones of the front and back mirrors. In some cases at least one of the mirrors is formed on one of the end faces of the gain medium. An end-pumped laser is one in which the pump radiation is provided to the gain medium through a back mirror specially designed to transmit high levels of the pump light to the gain medium and that still functions as a high reflectivity mirror at the lasing wavelength. For most such end-pumped solid-state lasers, there is a linear optical axis that passes from the pump source, through the front and back mirrors and through the resonant mode volume within the solid-state gain medium. Often, efforts are made to size and shape the beam of pump light produced by the semiconductor laser to closely approximate or match the mode volume in at least portions of the gain medium of the solid-state laser. Attempts to match the size and shape of the pump beam to the mode volume have the goal of maximizing coupling of the semiconductor laser""s output to the resonant mode within the gain medium, which generally maximizes output from the solid-state laser while reducing heat generation.
While an end-pumped configuration is very useful for efficiently coupling pump radiation from a semiconductor laser into the gain medium of a solid-state laser, there are practical difficulties in accomplishing the beam shaping and end coupling of the semiconductor laser to the gain medium. A typical crystal of Nd:YAG might measure one to two millimeters along its smaller axes and about one centimeter along its longest axis. The diameter of the gain mode defined within that gain medium is generally much smaller, on the order of two hundred to five hundred microns. High output semiconductor lasers generally have spatially distributed outputs derived from an array of emitters spaced over a distance of one-half centimeter to over one centimeter. Consequently, the optical output of a high output semiconductor laser will be an elongated beam having a major axis width on the order of one centimeter. Implementation of end pumping of a solid-state laser with a semiconductor laser therefore often requires that the laterally elongated output of the semiconductor laser be reshaped and focused to a small spot size to efficiently couple the semiconductor laser pump light through the small (xcx9c1 mmxc3x97xcx9c1 mm) end faces of the gain medium and into the gain volume. A sophisticated lens assembly might for example, accomplish this reshaping and focusing.
Economically providing the appropriate sophisticated lens assembly to function as the beam shaping optics that efficiently couple the elongated output from a semiconductor diode array into the end of a solid-state gain medium is difficult. Consequently, end-pumped lasers often use plural optical fibers to independently direct the output from the individual emitters of the semiconductor laser array into the end face of the gain medium. The use of plural independent fibers allows for the easy reshaping of the elongated output of the semiconductor laser array to a generally circular shape that is useful for end pumping a laser mode. Efficient coupling of the individual emitters to the pump face of the gain medium requires precise alignment of the optical fibers to the individual emitters. The alignment of the individual fibers to the individual emitters of the semiconductor laser bar is a difficult and expensive task. Regardless of whether a sophisticated lens assembly or fiber bundle coupling is used to couple the output of the semiconductor laser into the solid-state gain medium, the use of an end pumping configuration requires the use of optics that either are difficult to align and consequently expensive or are readily useable but expensive. End-pumped laser configurations also require at least one special cavity mirror adapted for transmitting pump radiation to the gain medium while still functioning acceptably as a cavity mirror.
End-pumped lasers are also constrained as to the extent by which they can be scaled. Such scaling would be desirable, for example, in obtaining higher levels of power output from the solid-state laser by coupling additional pump sources or greater pump intensity to the gain medium. End-pumped lasers couple all pump radiation into the solid-state gain medium through a portion of one of the small end faces of the gain medium. This presents two distinct sets of problems. First, there are problems to be overcome in coupling multiple semiconductor pump lasers to a solid-state laser in an end-pumped configuration. As discussed above, the mismatch in sizes between the spatially extended emitter region of the semiconductor laser and the gain volume requires special optics. Such optics might accommodate only a single semiconductor laser array and are not readily modified to accommodate multiple semiconductor laser arrays. Consequently, there may be physical space limitation problems that make it impractical to use pump sources to create higher pumping intensities in end-pumped lasers. A second problem is that the scalability of end-pumped solid-state lasers is fundamentally limited because all of the pump power passes through a single, confined portion of an end face of the solid-state gain medium. The total incident power that may be applied to the gain medium is limited by the fracture energy density of the gain medium. Application of greater pump intensities to the end face of the gain medium can cause the gain medium to fracture. In addition, much of the thermal stress on the solid-state gain medium occurs within a limited volume at the face of the gain medium. Even when the gain medium is not a strong absorber of the pump light, as is the case with Nd:YAG, the highest level of thermal buildup will be adjacent the entrance face of the gain medium. For gain media that exhibit significant thermal lensing effects, such a localization of heat input is particularly undesirable and detrimental to beam quality. The localized thermal loading associated with end pumping means that end-pumped lasers will generally be more limited as to the levels of optical pumping power that can be coupled into the solid-state gain medium in a reliable manner.
There has accordingly been a need to provide a solid-state laser system that is more readily scalable and which is optically less complex. Side pumped lasers offer some promise but, as is discussed briefly above, side pumped solid-state lasers tend to not effectively utilize input pump radiation. One alternative to this problem has been presented in the work of A. J. Alcock and J. E. Bernard as described in U.S. Pat. No. 5,315,612, entitled xe2x80x9cHigh Efficiency Transversely Pumped Solid-State Slab Laser,xe2x80x9d and in Bernard, et al., High -Efficiency Diode Pumped Nd:YVO4 Slab Laser, 18 Optics Letters 968-970 (June 1993) (together hereinafter referenced as the Alcock patent). The Alcock patent describes a side pumped solid-state laser that uses a very high-absorption coefficient gain medium, neodymium yttrium vanadate or Nd:YVO4, so that substantially all of the input pump radiation is absorbed within 250 microns of the surface of the gain medium. Because the absorption of the Nd:YVO4 gain medium is so high, the Alcock patent teaches that it is necessary to use a grazing angle geometry for the optical path within the gain medium. The cavity mirrors for the Alcock patent""s laser are positioned so that the resonant laser modes are internally reflected off one of the side faces of the gain medium at a grazing angle of incidence. Pumping is accomplished using a semiconductor laser having an extended output that is directed through a portion of the side face of the gain medium from which the laser mode is internally reflected. High levels of pump radiation couple into the resonant laser modes because the low angle between the gain mode volume and the side of the gain medium means that the pump face intersects an enlarged mode cross-section through the gain volume.
The solid-state laser described in the Alcock patent utilizes side pumping in a manner that reduces the incident pump energy density and distributes the thermal load associated with pumping over a larger surface area of the gain medium than is typical of an end-pumped laser. In this manner the Alcock patent""s laser overcomes some of the problems associated with the end pumped solid-state laser described above. On the other hand, the geometry of the Alcock patent""s laser introduces a number of different problems. A first problem with the geometry of the Alcock patent""s laser is that the Alcock laser is not readily compatible with high repetition rate pulsed laser operation of the type typical of Q-switched solid-state lasers. The gain medium of the Alcock laser is heated by the pumping radiation from the semiconductor laser to produce strong thermal lensing effects at the surface of the gain medium through which pumping occurs. Variations in the thermal lens created at the pump surface of the Alcock laser""s gain medium can introduce an unacceptable level of variations into the performance of the Alcock laser under pulsed operating conditions. For low pulse repetition rates of less than 1 kHz, variations in the thermal lensing associated with the heating of the Alcock laser""s gain medium by the pump radiation do not produce unacceptable levels of variation. For repetition rates faster than 1 kHz, on the other hand, time variation in the thermal lens at the surface of the Alcock laser introduces variations in the output of the Alcock laser so that the output beam quality deteriorates to approximately one and one-half times the diffraction limited performance and significantly worse at higher repetition rates. The Alcock patent""s laser is thus ill suited to high repetition rate pulsed operation.
A second problem with the laser geometry described in the Alcock patent is that the laser geometry of the Alcock patent necessarily introduces high levels of diffraction losses to the solid-state laser system. When the grazing angle geometry taught by the Alcock patent is applied to the available sizes of a solid-state gain medium, the path of the laser""s gain mode within the medium enters and exits the gain medium very near the edges of the side face of the gain medium. As such, the gain mode of the laser suffers significant diffractive losses through the side face of the gain medium. The level of diffractive losses experienced for practical lasers attempting to adopt the Alcock patent""s design are sufficiently high as to limit the optical output power from such lasers.
It is accordingly an object of some aspects of the present invention to provide a laser system incorporating a solid state laser that is comparatively simple and which is capable of being pumped with increased levels of optical power.
It is an object of other aspects of the present invention to provide a side pumped solid state laser having an efficient coupling of pump radiation into a gain volume spaced from a pump face of the gain medium.
These and other objects may be accomplished in a solid state laser that includes a high absorption coefficient solid state gain medium such as Nd:YVO4 that is side pumped with a semiconductor laser diode array. The resonant cavity of the solid state laser is preferably positioned so that the gain mode is spaced from the face of the laser through which the laser is pumped by a distance sufficient to reduce diffraction losses but sufficiently near to allow coupling of pump light into the gain mode. The gain medium, the doping level of the gain medium, and the operating temperature of the pump laser are selected to efficiently couple pump light into the gain mode.
According to one aspect of the present invention, a laser system includes a laser cavity having an optical axis passing through the laser cavity. The laser system includes a solid state gain medium having a width in a transverse direction, where the solid state gain medium has a absorption depth at a pumping wavelength less than the width in the transverse direction and the optical axis passing through the solid state gain medium. At least one semiconductor laser having a characteristic output wavelength is positioned to direct a beam of light to the solid state gain medium in a side-pumping configuration. The laser cavity defines a laser mode of the solid state laser positioned at a mode depth so that the laser mode is spaced from a surface of the gain medium.
Another aspect of the invention provides a laser system with a lasing medium for producing a laser beam of a desired wavelength. At least one diode laser directs a pumping beam along a pumping optical path for side-pumping the lasing medium and producing a population inversion in the lasing medium. Optical elements for forming a resonant cavity are provided around the lasing medium defining a set of modes and for producing the laser beam. The optical elements are disposed to position the optical set of modes within the lasing medium and the at least one diode laser provides a pump wavelength different from a peak of absorption wavelength of the lasing medium.
Still another aspect of the invention provides a laser system with a lasing medium for producing a laser beam of a desired wavelength. At least one diode laser directs a pumping beam along a pumping optical path for side pumping the lasing medium and producing a population inversion in the lasing medium. Optical elements for forming a resonant cavity are provided around the lasing medium defining a set of modes and for producing the laser beam. The optical elements are disposed to position the set of modes within the lasing medium, spaced from all sides of the lasing medium, and a width of the set of modes is between about one half and two times an absorption length of the lasing medium at a pumping wavelength.
Another aspect of the present invention provides a laser system including a laser cavity defined by at least two optical elements and having an optical axis passing through at least a first optical element, the optical elements defining a set of laser modes. The laser system includes a solid state gain medium having a transverse dimension and an absorption, where the optical axis passes through the solid state gain medium. A blocking element is positioned adjacent the optical axis between the solid state gain medium and the first optical element. At least one semiconductor laser having an output wavelength is positioned to direct a beam of light to the solid state gain medium in a side pumping configuration. The blocking element is positioned to limit gain of one or more of the set of laser modes.
The invention also provides a method of assembling a laser system including a solid state laser. Optics and a solid state gain medium are provided to define a solid state laser cavity having a set of laser modes, where the set of laser modes has a position within the gain medium and the gain medium has an absorption depth. At least one semiconductor laser having an output wavelength is positioned to side pump the gain medium to produce a solid state laser output. The semiconductor providing a pump beam has a size. The method includes a step of optimizing a configuration of the solid state laser by at least one of (1) varying the position of the set of laser modes within the gain medium, (2) varying the output wavelength of the semiconductor laser, and (3) varying the size of the pump beam.